Road vehicles with wheel suspension systems which contain an active or semi-active level or height adjustment system for the vehicle superstructure offer various benefits in comparison with vehicles with a passive wheel suspension. Thanks to a constant ground clearance, the driving behavior can be improved as the spring compression and extension movements of the wheels are independent of the vehicle load. Also it is possible for example, in city driving i.e. at low speeds, to raise the vehicle superstructure from the ground and hence increase the ground clearance in order for example to be able to drive without problems over road humps, curbstones or underground car park entrances with a high ramp angle. At higher drive speeds the vehicle superstructure can be lowered using the ride height adjustment system in order for example to reduce the drag of the vehicle. In vehicles, in particular in cars, the movements of the vehicle superstructure are normally mainly damped hydraulically, for example by means of passive shock and vibration dampers working with hydraulic oil. Furthermore hydraulic vibration dampers are also known, the damping characteristics of which can be altered during operation. The ride height adjustment systems currently available on the market for vehicles are relatively costly and have a high weight. In addition they require a large installation space and use relatively large quantities of energy for operation.
For example DE 10 2006 055 757 A1 discloses a self-pumping air spring and damper unit working with compressed air, with automatic ride height adjustment, for a vehicle chassis. The air spring and damper unit has a pump which is driven by the changing distance between the pivot points of the spring and damper unit on the vehicle superstructure and chassis respectively as a result of the spring compression and extension movements of the vehicle, and generates an increase in air pressure whereby the air spring and damper unit retains an established ride height when the load on the vehicle changes. The pump is substantially formed from two pump parts which are moveable relative to each other and reduce a compression chamber, of which one pump part is connected with the one pivot point and the other pump part with the other pivot point such that on the spring compression and extension movements of the vehicle, the changing distance between the pivot points is transmitted to the mutual spacing of the pump parts. The air spring and damper unit furthermore has a passive, pneumatic vibration damper formed by two spring chambers which contain compressed air and which can be connected via a throughflow choke valve, wherein the compressed air is exchanged between the spring chambers damped by the choke valves on the spring compression and extension movements of the vehicle. One of the spring chambers is delimited by a roller bellows which rolls on the contour of a rotationally symmetrical rolling cylinder. The compression chamber of the pump can be connected fluid-conductively with one of the spring chambers via a check valve so that on a compression stroke of the pump or on spring compression, air is pumped into a spring chamber by the pump. On spring extension, air is discharged from the spring chamber via a discharge device also provided, which substantially comprises a discharge cylinder and a discharge piston. The pump and the discharge device are matched to each other such that a constant vehicle height is always maintained irrespective of the vehicle load due to the spring compression and extension movements of the vehicle superstructure.
Furthermore DE 103 27 485 A1 discloses an active pneumatic suspension system for a motor vehicle with an air spring and a passive vibration damper. The air spring has a spring chamber filled with compressed air, wherein the compressed air can be compressed or expanded via a hydraulic fluid which can be pumped into the spring chamber, and is separated from the compressed air in the spring chamber by means of an elastic membrane. The hydraulic fluid is pumped by means of an electrically operated hydraulic pump from a hydraulic accumulator into the spring chamber and vice versa.
DE 100 34 603 A1 furthermore discloses a pneumatic air spring and damping element which has two spring chambers separated by a choke element, of which chambers at least one is variable in its volume. In the case that both spring chambers are variable in volume, an additional spring element in the form of a coil spring is connected in parallel with the air spring and damper element.
Furthermore DE 25 38 805 A1 discloses a vehicle wheel suspension with a telescopic shock absorber and a coil spring connected in parallel to the shock absorber, wherein a tubular flexible membrane element is provided outside the shock absorber to form a chamber of variable gas volume. To control the gas volume in the chamber, gas supply devices are provided for optional supply of a gas, for example compressed by means of a compressor, to and from the chamber.
DE 1 216 126 discloses a hydraulic telescopic shock absorber connected with a support suspension for vehicles, wherein the support suspension for example can be formed as coil springs or air springs arranged coaxially to the shock absorber. The support suspension load is supported via a moving wall on the damping fluid which is otherwise enclosed by solid walls on all sides.
In operation of the known self-pumping devices with an air spring which provides a height adjustment function, in general damping forces are generated by the pumping of air into and out of the spring chamber of the air spring via corresponding valves. In addition the spring stiffness of the air springs changes on a height adjustment process, under constant air pressure, as a result of pumping of air into and out of the spring chamber of the air spring. Both the temporary generation of additional damping forces and the change in spring stiffness as a function of ride height are undesirable as they can lead in particular to perceptible losses of driving comfort.